The skin of human beings is variously coloured with remarkable individual differences occurring even within members of the same race. The epidermis or upper layer of the skin consists essentially of two major layers. The viable basal layer exists at the bottom of the epidermis in contact with the dermis, while the dead corneal or horny layer exists above the basal layer and extends upward to the surface of the skin.
The colour of human skin is determined by melanin, a biopolymer pigment manufactured by special dendritic cells known as melanocytes residing mostly below or between the basal cells of the epidermis. Melanocytes have been characterized as unicellular "glands" having long, thin, branching, streamer-like dendrites or arms that worm their way between the epidermal cells in their immediate vicinity creating a constellation of epidermal cells around each melanocyte. Melanin is produced in the melanosome region and once produced travels to the dendrites of the melanocyte. Epidermal cells in contact with the melanin-laden dendrites actually phagocytose the tips of the dendrites and transfer the melanin to the surrounding epidermal cells. Once inside the epidermal cells, melanin granules tend to move above the cell nucleus forming a type of shroud over it. This orientation of melanin supports other evidence that it exists to protect cells from damaging ultraviolet rays. In fact, melanin production is stimulated by UV irradiation.
The biochemical process responsible for the production of melanin is caused by the action of an enzyme called tyrosinase which triggers a cascade of biosynthesis. Tyrosinase causes the oxidation of the substrates tyrosine and levodopa to dopaquinone followed by subsequent polymerization of numerous intermediates into melanin. The exact mechanism of melanin biosynthesis is a complex process however the importance of tyrosinase in propelling the overall process is undisputed. M. M. Wick, V. J. Hearing and H. Rorsman, Biochemistry of Melanization, pp. 251-256 (19).
Although primary regulation of melanin production is via genetic controls, environmental factors may also play an important role in synthesis. As noted above, exposure to sunlight or other UV radiation can stimulate the melanocytes to produce melanin, hence the so-called "tanning" reaction. Current research suggests that an inhibitor of tyrosinase activity is destroyed by UV radiation thereby allowing for the rapid stimulation of pigment production. Melanin production can also increase in response to hormone fluctuations associated with child bearing or the use of birth-control pills.
Normal pigmentation of the skin surface is uniform, however localized, excessive pigmentation can occur and such colorization is collectively referred to as hyperpigmentation. Hyperpigmentation encompasses a wide array of afflictions all of which are accompanied by increased melanin production. Hyperpigmentation of the human skin may include skin blemishes or disorders including freckles, senile lentigo, liver spots, melasma, brown or age spots, vitiligo, sunburn pigmentation, post-inflammatory hyperpigmentation due to abrasion, burns, wounds, dermatitis, phototosic reaction and other similar small, fixed pigmented lesions. In addition and from a cosmetic standpoint, it may often be desirable to decolorize what is considered normally pigmented skin to increase "fairness" or to blend hyperpigmented regions into that of the surrounding normal skin. Regardless of the type of hyperpigmentation, it is almost always viewed as cosmetically undesirable and often to the point of being psychologically disabling to the patient.
Accordingly, a number of prior art methods and compositions have been developed in an attempt to depigment the skin. Most of these prior art attempts have focused on skin-bleaching compositions such as sodium hypochlorite, hydroquinone, monoethyl ether, ammoniated mercury, zinc peroxide, mercurous chloride and bi-chloride of mercury. These compounds are often disadvantageous due to sensitization, irritation and lack of predictable results. In addition, methods incorporating such compounds are generally ineffective or at the most short lived since they do not address the cause of pigmentation but rather the effects of pigmentation.
To be effective, any treatment must inhibit at the melanocytes either the production of the enzyme tyrosinase since that enzyme is ultimately responsible for conversion of tyrosine into melanin or, the production of intermediates during melanogenesis.
It is known that certain compounds effectively inhibit synthesis of melanin. For example, recent studies have shown that 6-nitroquipazine, a serotonin uptake antagonist, inhibits melanin synthesis without affecting tyrosinase activity. See "Inhibition of Melanization in Human Melanoma Cells by a Serotonin Uptake Inhibitor" by M. McEwan, P. G. Parsons in J. INVESTIGATIVE DERMATOLOGY, 1987, Vol. 89, pp. 82-86.
Other studies have shown that tyrosinase activity and consequently melanin production can be decreased and eventually halted by various histamine agonists. See "Regulation of Tyrosinase Expression and Activity in Human Melanoma Cells via Histamine Receptors" by M. T. McEwan and P. G. Parsons in J. INVESTIGATIVE DERMATOLOGY, 1991, Vol. 97, pp. 868-873.
Although various prior art depigmentation compositions have included melanin synthesis inhibiting compounds, they are for the most part ineffective since they don't truly inhibit the activity of tyrosinase or sufficiently interfere with melanin biosynthesis. For example, in U.S. Pat. No. 4,919,291 (Hatae) kojic acid and its esters has been combined with Vitamin C in a topical composition for use as a depigmentation cream. Kojic acid is known to inhibit synthesis of melanin. U.S. Pat. No. 3,856,934 (Kligman) discloses a topical composition combining retinoic acid and corticosteriod to synergistically create a melanin inhibiting topical composition. U.S. Pat. No. 4,096,240 (Mathur) discloses combining Niacinamide with a UV absorbing sunscreen for use in a topical composition to "retard" melanin dispersion or distribution into the epidermis. Although each of above noted compositions include melanin inhibiting ingredients in various topical oils, lotions or emulsions, none of the compositions have been found to be effective since the water-solubility of the melanin inhibiting compound prevents delivery into the basal cell region of the epidermis. These penetrating lotions and oils and active ingredients are invariably adsorbed into the upper layer of the epidermis and dispersed well before they can reach the basal cell layer to interfere with melanin production.
In addition, many carriers can cause an adverse reaction in the user. For example, while dimethyl sulfoxide (DMSO) has found wide use as a penetrant carrier it can also cause extreme skin irritation, redness, itching and scaling. Water soluble organic or inorganic zinc salts have also been proposed as pharmaceutical vehicles for enhancing penetration and retention of drugs into the skin. These carriers have likewise been met with mixed success since they are not inert and often interfere with the action of the active ingredient or cause allergic reactions to the patient. Further, none of the prior art penetrant carriers deliver the active ingredient to the basal cell region intact. The water soluble characteristics of both the carriers and active ingredients results in premature dispersion within the skin well before the melanocyte region. Thus, despite the development of various melanin inhibiting compounds, prior art methods and compositions only provide short term relief with relatively large doses being required to compensate for excessive adsorption into the surrounding epidermal layers.
It is known to administer DNA repair enzymes in an active form to living cells in situ via topical compositions. In U.S. Pat. No. 5,077,211 (Yarosh) enzymes are first encapsulated within liposomes and the liposomes are subsequently suspended in a topical vehicle. The liposomes provide a non-toxic means for encapsulation and can be further modified to bind to specific sub-populations of cells. Liposomes have also been previously used to encapsulate melanin for introduction into non-pigmented human fibroblast. See "Polyethylene-Glycol-mediated Delivery of Liposome-entrapped Pigments into FibroblaSts: Experimental Pigment Cells as Models for Mutator Phenotypes" by S. Schmitz, T. M. Allen and K. Jimbow in CANCER RESEARCH, 1992, Vol. 52, pp. 6638-6645.
A need has therefore existed within the art for a method and composition of skin depigmentation which possesses true melanin synthesis inhibiting activity and one which can be effectively delivered through the skin via a topical composition for eventual adsorption into the basal cell region of the skin thereby promoting interference of melanin production.